The contrapositive is the converse of the inverse (or the inverse of the converse if you like):
If not , then not , symbolically:
Presuming the original statement to be true (that is you have a cell phone that beeps only if the battery is low), then the contrapositive is always true. If you don't need to charge the battery then the phone isn't beeping. On the other hand, the converse and the inverse are not necessarily true. It is possible that the battery is so far drained that there is insufficient power to make the phone beep.
My calculator said it, I believe it, that settles it